Micro pick up array pivot mount design for strain amplification

ABSTRACT

Systems and methods for aligning a transfer head assembly with a substrate are disclosed. In an embodiment a pivot mount is used for generating a feedback signal in a closed-loop motion control system. In an embodiment, the pivot mount includes primary spring arms and secondary spring arms extending between a pivot platform and a base of the pivot mount. The secondary spring arms are characterized by a lower stiffness than the primary spring arms, and strain sensing elements are located along the secondary spring arms.

BACKGROUND

Field

Embodiments relate to an alignment system. More particularly embodiments relate to a micro pick up array pivot mount with integrated strain sensing elements for aligning an electrostatic transfer head array with a target substrate.

Background Information

The feasibility of commercializing miniature devices such as radio frequency (RF) microelectromechanical systems (MEMS) microswitches, light-emitting diode (LED) display systems, and MEMS or quartz-based oscillators is largely constrained by the difficulties and costs associated with manufacturing those devices. Miniaturized device manufacturing processes typically include processes in which miniaturized devices are transferred from one wafer to another. In one such implementation, a transfer wafer may pick up an array of miniaturized devices from a donor wafer and bond the miniaturized devices to a receiving wafer. Methods and apparatuses for aligning two flat surfaces in a parallel orientation have been described, and may be applied to miniaturized device transfer.

SUMMARY

A pivot mount and transfer tool are described. In an embodiment a pivot mount includes a pivot platform, a base, a primary spring arm, a secondary spring arm characterized as having a lower stiffness than the primary spring arm, and a strain sensing element along the secondary spring arm. The primary spring arm is fixed to the pivot platform at a primary inner root, fixed to the base at a primary outer root, and characterized by a corresponding primary axis length spanning between the primary outer root and primary inner root. The secondary spring arm is fixed to the pivot platform at a secondary inner root, fixed to the base at a secondary outer root, and characterized by a corresponding secondary axis length spanning between the secondary outer root and secondary inner root. The relative stiffness of the primary spring arm and secondary spring arm may be selected by adjusting the length, width, or thickness of the spring arm designs. For example, the primary axis length may be greater than the secondary axis length. An average width along the primary axis length may be wider than an average width along the secondary axis length. The primary spring arm and secondary spring arm may also share the same average thickness along their respective axial lengths. In an embodiment, the primary spring arm and the secondary spring arm are formed of the same material. For example, each may be formed from the same silicon substrate, and each may be integrally formed. In an embodiment, the secondary spring arm has a lower average thickness along its axial length than the primary spring arm. In this manner the relative stiffness can be selected by modulating thickness of the spring arms. In an embodiment, the relative stiffness of the primary spring arms and secondary spring arms is modulated by selectively etching the secondary spring arms. In an embodiment, the relative stiffness of the primary spring arms and secondary spring arms is modulated by adding one or more layers with differentiated features. The pivot mount may include a plurality of primary spring arms and secondary spring arms.

The primary spring arms and/or secondary spring arms may include one or more switch-backs along an axial length. In an embodiment, a secondary spring arm includes a switch-back along the secondary axis length such that a first beam segment and a second beam segment of the secondary spring arm immediately adjacent the switch-back are parallel to each other. In an embodiment, a first strain sensing element is at the first beam segment, and a second strain sensing element is at the second beam segment. First and second reference gages may also be located adjacent the first and second strain sensing elements at the first and second beam segments. In an embodiment, the second beam segment is longer than the first beam segment. The secondary spring arm may include a plurality of switch-backs along the secondary axis length. In an embodiment, the secondary spring arm includes a plurality of beam segments of a first length along the secondary axis length, and a beam segment of a second length longer than the first length along the secondary axis length. The primary spring arm may also include a plurality of switch-backs along the primary axis length. In an embodiment, a pair of secondary spring arms is laterally between a pair of primary spring arms, with each secondary spring arm characterized as having a lower stiffness than each of the primary spring arms.

The pivot mount may be integrated into a transfer tool. In an embodiment, the transfer tool includes an articulating transfer head assembly, a pivot mount, and a micro pick up array (MPA) mountable onto the pivot platform of the pivot mount. The MPA includes an array of transfer heads, such as electrostatic transfer heads. In an embodiment, each transfer head has a localized contact point characterized by a maximum dimension of 1-100 μm in both x- and y-dimensions. In an embodiment, the pivot platform includes a plurality of compliant voltage contacts, and the micro pick up array includes a plurality of voltage contacts arranged to mate with the plurality of compliant voltage contacts of the pivot platform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is perspective view illustration of a mass transfer tool in accordance with an embodiment.

FIG. 2 is a perspective view illustration of a micro pick up array and pivot mount mounted onto a transfer head assembly in accordance with an embodiment.

FIG. 3 is an exploded cross-sectional side view illustration of a transfer head assembly, pivot mount, and micro pick up array in accordance with an embodiment.

FIG. 4 is a schematic top view illustration of a micro pick up array in accordance with an embodiment.

FIGS. 5A-5E are cross-sectional side view illustrations of a method of forming a pivot mount including compliant voltage contacts in accordance with an embodiment.

FIG. 6A is a top view illustration of a pivot mount and a primary axial length of a primary spring arm in accordance with an embodiment.

FIG. 6B is a top view illustration of a pivot mount and a secondary axial length of a secondary spring arm in accordance with an embodiment.

FIG. 6C is a close-up top view illustration of Detail B in FIG. 6B in accordance with an embodiment.

FIG. 6D is a close-up top view illustration of Detail B in FIG. 6B in accordance with an embodiment.

FIG. 6E is a close-up top view illustration of Detail D in FIG. 6D in accordance with an embodiment.

FIGS. 6F-6H are cross-sectional side view illustrations along section A-A in FIGS. 6A-6B of a method of modulating stiffness by reducing layer thickness in accordance with an embodiment.

FIGS. 6I-6J are cross-sectional side view illustrations along section A-A in FIGS. 6A-6B of a method of modulating stiffness with layer build up in accordance with an embodiment.

FIG. 7 is a top view illustration of a pivot mount including various structural features and electrical routing in accordance with an embodiment.

FIG. 8A is an illustration of strain components in a body.

FIG. 8B is an illustration of strain components in a thin structure.

FIG. 9 is an illustration of a spring arm under pure bending in accordance with an embodiment.

FIG. 10 is an illustration of a spring arm under simultaneous bending and torsion in accordance with an embodiment.

FIGS. 11A-11B are perspective view illustrations of a pivot platform of a pivot mount deflected with a uniform z displacement in accordance with an embodiment.

FIG. 11C is a perspective view illustration of strain modeling for normal strain in the x direction for a pivot platform deflected with a uniform z displacement in accordance with an embodiment.

FIG. 11D is a perspective view illustration of strain modeling for normal strain in the y direction for a pivot platform deflected with a uniform z displacement in accordance with an embodiment.

FIG. 11E is a perspective view illustration of strain modeling for equivalent strain magnitude for a pivot platform deflected with a uniform z displacement in accordance with an embodiment.

FIG. 11F is a perspective view illustration of strain modeling for surface shear strain for a pivot platform deflected with a uniform z displacement in accordance with an embodiment.

FIG. 12 is a schematic illustration of an idealized beam of length L with one end fixed and subject to simultaneous transversely applied force and bending moment at the free end and with zero slope boundary conditions at both ends.

FIG. 13 depicts shear force and bending moment diagrams for an idealized beam of length L with one end fixed and subject to simultaneous transversely applied force and bending moment at the free end and with zero slope boundary conditions at both ends.

FIG. 14 is a top view illustration with detail view of a strain amplifying secondary spring arm structure with each beam segment labeled sequentially starting at the inner root to the outer root in accordance with an embodiment.

FIG. 15A depicts a bending moment diagram along the axial length of the secondary spring arm structure of FIG. 14 in accordance with an embodiment.

FIG. 15B depicts a bending moment diagram along the axial length of the secondary spring arm structure of FIG. 14 with an applied loading of equal magnitude and opposite sense to the loading presumed in FIG. 15A in accordance with an embodiment.

FIG. 16 is a top view illustration with detail view strain amplifying secondary spring arms including correlated strain sensors in accordance with an embodiment.

FIG. 17 is top view illustration of a pivot mount in accordance with an embodiment.

FIG. 18A is a schematic illustration of a control scheme for regulating a transfer head assembly in accordance with an embodiment.

FIG. 18B is a schematic illustration of a method of generating a synthesized output signal in accordance with an embodiment.

FIG. 18C is a schematic illustration of a method of generating a synthesized output signal in accordance with an embodiment.

FIG. 19 is a flowchart illustrating a method of aligning a micro pick up array relative to a target substrate in accordance with an embodiment.

FIG. 20 is a schematic illustration of a computer system in accordance with an embodiment.

DETAILED DESCRIPTION

Embodiments describe a pivot mount including a base, a pivot platform, primary spring arms extending between the pivot platform and the base, and secondary springs arm extending between the pivot platform and the base. The secondary spring arms are characterized as having a lower stiffness than the primary spring arms, and strain sensing elements are located along the secondary spring arms. In this manner, when the pivot mount is moved in a direction orthogonal to a contact surface of the pivot platform normal strain is created at the surface of the spring arms. Since the secondary spring arms have a lower stiffness than the primary spring arms, the secondary spring arms undergo more strain for a given pivot platform displacement. By locating the strain sensing elements along the secondary spring arms a larger amount of strain is measured. Thus, in an embodiment, the primary spring arms provide stiffness for the pivot mount, for example to support the MPA and achieve the operational amount of deflection during the pick and place operations of the transfer tool, while the secondary spring arm provides strain amplification, and therefore signal amplification, enabling higher sensitivity of the pivot mount.

The pivot mount can be coupled to an articulating head assembly of a mass transfer tool for accurate and repeatable alignment in 6 spatial degrees of freedom between the transfer tool and a target substrate. When accurately aligning two planar surfaces, lateral (x and y) and rotational (θz) alignments are relatively straightforward to achieve through use of a high-precision x-y stage and rotationally-positioned substrate chucks. The remaining three degrees of freedom, θx, θy, (or, “tilt” and “tip”) and z are difficult to independently control. Any changes to the tip and tilt angle necessarily change the distance z to any point not located at the center of rotation. While parallelism between two planes can be accomplished through use of a passive pivot mount, the pressure distribution between the two planar surfaces will not be centered or uniform unless the two surfaces were parallel to begin with. A transfer tool including a pivot mount in accordance with embodiments described herein may redistribute the pressure distribution to achieve a uniform pressure field. By placing strain sensing elements (strain gages) at high-strain locations on the pivot mount secondary spring arms, a feedback signal of the position error can be generated and input to the transfer tool for operation in a closed-loop motion control system. Because strain is related to the stress state through Hooke's Law, both displacement and forces acting on the pivot mount can be known by measuring strain.

In one aspect, embodiments describe a pivot mount configuration including primary spring arms to provide stiffness for the pivot mount, and secondary spring arms to provide strain amplification, and therefore signal amplification, enabling higher sensitivity of the pivot mount. In an embodiment, the length of the secondary spring arms is less than the length of the primary spring arms. In such a configuration where an amount of deflection for the primary and secondary spring arms is equivalent for a given pivot platform displacement, the strain resulting from the given loading is greater in the secondary spring arms than in the primary spring arms. As a result, the sensitivity of the pivot mount as a displacement sensing device is increased.

In one aspect, embodiments describe a pivot mount configuration that achieves a high strain sensing sensitivity and generates a feedback signal with a high signal to noise ratio. As a result the pivot mount can provide a position feedback signal with increased effective resolution to the transfer tool. By locating the strain sensing elements near the inner and outer roots or on opposite sides of switch-backs in an axial length of a spring arm, equal and opposite strain responses are measured. In this manner a strain signal for a given platform displacement may be effectively doubled. Such a configuration can also reduce noise for a given strain signal. Due to the differential sensing at the inner and outer roots and the switch-backs the measured noise is effectively canceled. Accordingly, higher strain sensing sensitivity may be accomplished with a higher signal to noise ratio, and an increased effective resolution of the position feedback signal may be provided to the transfer tool.

In another aspect, embodiments describe pivot mount spring arm configurations that minimize the torsion applied to a spring arm at the roots where a spring arm is fixed to a pivot platform at one end and fixed to a base at another end. This creates a more uniform bending moment in the high strain regions of the spring arm with reduced strain variation and torsion in the spring arms, which allows the strain sensing elements to be located in the high strain regions near the roots. By comparison, in other configurations with spring arms that undergo both bending and torsional loading, the area of maximum strain may include both bending and torsion. Torsion in the spring arms is parasitic to surface strain sensing since it manifests as strain at the surface of the spring arm having components in both the x and y directions. Because the total strain energy distributed through the spring arms is constant for a given pivot platform displacement, the presence of strain components perpendicular to the strain sensing elements reduces the ratio of strain components that are aligned with the strain sensing elements. As a result, strain sensing elements located near areas of torsion may produce a lower effective feedback signal and sensitivity. In an embodiment, a pivot mount is arranged to create boundary conditions at the roots of the spring arms with a uniform bending moment, in which strain is substantially perpendicular to the roots and substantially parallel to strands in the strain sensing elements, which may be parallel to axial lengths of the spring arms in the high strain regions. Such a configuration directs substantially all of the strain energy from a given pivot platform displacement into strain components aligned with the strain sensing elements. As a result, higher strain may be measured and sense feedback signal strength may be increased for a given pivot platform displacement. Reduction of the torsional moment at the roots may additionally allow more freedom in stiffness requirements of the spring arms. In turn, reduced stiffness requirements allow for greater bending, resulting in increased normal strain at the surface of the spring arms and sense feedback signal strength.

In another aspect, reduction of the torsional moment applied to the spring arms at the roots may also increase the effectiveness of the reference gage(s) positioned adjacent the strain sensing elements. In an embodiment, each strain sensing element is located in a high strain region of a spring arm that sees only normal strain at the surface in the gage direction of the strain sensing element and sees no normal strain at the surface lateral to the gage direction. This allows the location of a reference strain gage adjacent to each strain sensing element with the result that the reference gages do not see strain caused by mechanical loading of the pivot platform. This in turn allows the reference gages to compensate for temperature variations in the system, and increase the signal to noise ratio. Since the strain sensing elements and reference gages are adjacent, they are exposed to the same temperature, meaning the thermal strain is identical in both a strain sensing element and a corresponding reference gage. Since the reference strain gages are not subjected to strain resulting from mechanical load, any strain signal they produce can be attributed to temperature (as noise), which is then subtracted as background noise from the strain measured by the adjacent strain sensing element. In an embodiment, strands in the reference gages are oriented perpendicular to strands in the strain sensing elements. In such a configuration, the normal strain at the surface of the spring arms is substantially parallel to the strands in the strain sensing elements, and perpendicular to the strands in the reference strain gages. Thus, by reducing the torsional moment and creating uniform bending moments in the spring arms in which normal strain at the surface of the spring arms is substantially perpendicular to the roots, the reference strain gages may be more accurate and a higher strain sensing sensitivity may be accomplished with a higher signal to noise ratio.

In another aspect, embodiments describe an arrangement of strain sensing elements into distributed, correlated pairs. In an embodiment, a correlated pair of strain sensing elements (and reference gages, if present) forms a sensor. The sensors may also be arranged into distributed, correlated sensors. In an embodiment, each sensor includes one or more correlation sensors. In these manners, the loss of a strain sensing element or sensor does not prohibit use of the pivot mount, and the lifetime of the pivot mount use with a transfer tool can be extended. In essence, redundancy is obtained by having pairs of the same signal. For example, a correlated pair of strain sensing elements or sensors may each sense a same z-deflection. In another situation, a correlated pair may sense a same or equal but opposite θx, θy, (or, “tilt” and “tip”). In either situation, the loss of one of the correlated strain sensing element or sensor may reduce the overall signal to noise ratio generated from the pivot platform, yet the remaining signal to noise ratio remains adequate for operation of the transfer tool.

In yet another aspect, embodiments describe a pivot mount with compliant voltage contacts, for providing a low contact resistance connections of the voltage contacts to a micro pick up array (MPA) that is mounted onto the pivot platform of the pivot mount. The compliant voltage contacts may protrude from the pivot platform such that they are elevated above the pivot platform, yet are compliant such that they exert a pressure upon the MPA contacts when the MPA is clamped onto the pivot mount pivot platform, for example, using an electrostatic clamp contact on the pivot mount platform.

Referring to FIG. 1, a perspective view of a mass transfer tool is shown. Mass transfer tool 100 may include a transfer head assembly 200 for picking up an array of micro devices from a carrier substrate held by a carrier substrate holder 104 and for transferring and releasing the array of micro devices onto a receiving substrate held by a receiving substrate holder 106. Embodiments of a mass transfer tool are described in U.S. Patent Publication No. 2014/0071580, titled “Mass Transfer Tool”, filed on Sep. 7, 2012. Operation of mass transfer tool 100 and transfer head assembly 200 may be controlled at least in part by a computer 108. Computer 108 may control the operation of transfer head assembly 200 based on feedback signals received from various sensors, strain sensing elements, reference gages located on a pivot mount. For example, transfer head assembly 200 may include an actuator assembly for adjusting an associated MPA 103 with at least three degrees of freedom, e.g., tipping, tilting, and movement in a z direction, based on feedback signals received from sensors associated with a pivot mount that carries MPA 103. Similarly, the carrier substrate holder 104 and receiving substrate holder 106 may be moved by an x-y stage 110 of mass transfer tool 100, having at least two degrees of freedom, e.g., along orthogonal axes within a horizontal plane. Additional actuators may be provided, e.g., between mass transfer tool 100 structural components and transfer head assembly 200, carrier substrate holder 104, or receiving substrate holder 106, to provide movement in the x, y, or z direction for one or more of those sub-assemblies. For example, a gantry 112 may support transfer head assembly 200 and move transfer head assembly 200 along an upper beam, e.g., in a direction parallel to an axis of motion of x-y stage 110. Thus, an array of electrostatic transfer heads on MPA 103, supported by transfer head assembly 200, and an array of micro devices supported by a carrier substrate held by carrier substrate holder 104 may be precisely moved relative to each other within all three spatial dimensions.

Referring to FIG. 2, a perspective view of a transfer head assembly 200 is shown in accordance with an embodiment. A transfer head assembly 200 may be used in combination with mass transfer tool 100 to transfer micro devices to or from a substrate, e.g., receiving substrate or carrier substrate, using MPA 103 which is supported by a pivot mount 300. More particularly, transfer head assembly 200 may provide for negligible lateral or vertical parasitic motion for small movements of MPA 103, e.g., motion less than about 5 mrad about a neutral position. Accordingly, transfer head assembly 200 may be incorporated in mass transfer tool 100 to adjust an MPA 103 relative to mass transfer tool 100. Thus, transfer head assembly 200 may be fixed to a chassis of mass transfer tool 100, e.g., at a location along an upper beam or support.

As illustrated, the pivot mount 300 may include a base 302, a pivot platform 304, a plurality of primary spring arms 306, and a plurality of secondary spring arms 307, and the MPA 103 supporting a transfer head array 115 is mounted on the pivot platform 304. In an embodiment, the transfer head array 115 is an electrostatic transfer head array 115, where each transfer head operates in accordance with electrostatic principles to pick up and transfer a corresponding micro device. In an embodiment each electrostatic transfer head has a localized contact point characterized by a maximum dimension of 1-100 μm in both the x- and y-dimensions. In an embodiment, the pivot mount 300 may communicate and send feedback signals to the mass transfer tool 100 through one or more electrical connections, such as a flex circuit 308. As described below, feedback may include analog signals from various sensors, strain sensing elements, reference gages that are used in a control loop to regulate actuation and spatial orientation of the transfer head assembly 200. In an embodiment, the feedback signals are sent to a position sensing module located near the pivot mount 300 to reduce signal degradation by limiting a distance that analog signals must travel from a strain sensing element to the position sensing module. In an embodiment, the position sensing module is located within the transfer head assembly 200.

Referring now to FIG. 3, an exploded cross-sectional side view illustration is provided of a transfer head assembly 200, pivot mount 300, and MPA 103. Generally, the pivot mount 300 is mounted onto the transfer head assembly 200. This may be accomplished using a variety of manners such as using tabs or lips to press the pivot mount against the transfer head assembly 200, bonding, vacuum, or electrostatic clamping. A deflection cavity 202 may be formed in the transfer head assembly 200 to allow a specified z-deflection distance of the pivot platform 200 along the z-axis.

As illustrated in FIG. 3, the pivot mount 300 may include channels 310 formed through a body of the pivot mount from a front surface 312 to back surface 314. Channels 310 may be used to form a variety of compliant features of the pivot mount 300, including defining the primary spring arms 306, secondary spring arms 307, and pivot platform 304, as well as the compliant voltage contacts 316, described in more detail in the following description. The compliant voltage contacts 316 may provide a low contact resistance connection to voltage contacts 120 of the MPA 103. In the embodiment illustrated, the compliant voltage contacts 316 protrude from the pivot platform such that they are raised above the pivot platform. Upon clamping the MPA 103 onto the pivot platform of the pivot mount 300 with the opposing electrostatic clamp contacts 318, 122, the compliant voltage contacts 316 exert a pressure upon the MPA contacts 120. Additional features may be located on or in the pivot mount 300. For example, strain sensing elements 320 (strain gages) and reference gages 340 may be located at high strain regions of the secondary spring arms 307, as described in further detail in the following description. Strain sensing elements 320 and reference gages 340 may also be located at high strain regions of primary spring arms 306.

Referring now to FIG. 4, a schematic top view illustration of an MPA 103 is shown in accordance with an embodiment. In an embodiment, an area of the electrostatic clamp contact 122 on a back side of the MPA is larger than an area of the transfer head array 115 on the front surface of the MPA. In this manner, the alignment and planarity across the transfer heads in the transfer head array 115 can be regulated by alignment of the transfer head assembly. In such an embodiment, a plurality of voltage contacts 120 for supplying an operating voltage to the transfer head array 115 is located outside the periphery of the transfer head array 115.

Referring now to FIGS. 5A-5E, cross-sectional side view illustrations are shown for a method of forming a pivot mount 300 including compliant voltage contacts 316. The processing sequence may begin with a commercially available silicon wafer 301 including a top oxide layer 330, and bottom oxide layer 332 as illustrated in FIG. 5A. While the following description is made with regard to a silicon wafer, embodiments are not so limited, and other suitable substrates can be used to form pivot mount 300, such a silicon carbide, aluminum nitride, stainless steel, and aluminum, amongst others. In an embodiment illustrated in FIG. 5B, the top oxide layer 330 is then removed, with bottom oxide layer 330 remaining. Referring to FIG. 5C, the top and bottom surfaces of the wafer 301 may then be oxidized further resulting in a top oxide layer 334, and bottom oxide layer 336 that is thicker than the previous bottom oxide layer 332 and thicker than the top oxide layer 334. For example, this may be accomplished with a wet thermal oxidation operation. Following the formation of oxide layers 334, 336 various layers may be formed over the top oxide layer 334 to form the strain gages 320, reference gages 340, electrostatic clamp contact(s) 318, and electrodes 317 for the compliant voltage contacts. In an embodiment, these various layers may be formed by one or more metal deposition processes. In an embodiment, the electrodes 317 for the compliant voltage contacts are thicker than other metallization layers used to form the strain gages 320, reference gages 340, and electrostatic clamp contact(s) 318. Referring to FIG. 5E, the bottom oxide layer 336 is removed and channels 310 are etched through the silicon wafer 301 and top oxide layer 334 to define the primary spring arms 306, secondary spring arms 307, pivot platform 304, and compliant voltage contacts 316. As shown in FIG. 5E, the contact surfaces including the electrodes 317 for the compliant voltage contacts 316 protrude from the pivot platform such that they are elevated above the surrounding pivot platform, including the strain gages 320, reference gages 340, and electrostatic clamp contact(s) 318. This may be the result of releasing residual stress within the silicon wafer 301 during formation of the channels 310. In an embodiment, the residual stress was created in the silicon wafer 301 during the oxidation and removal operation described and illustrated in FIGS. 5A-5C. In accordance with embodiments, the channels 310 forming the compliant voltage contacts 316 may assume a variety of configurations such as switch-backs or a winding contour. In an embodiment, the channels forming the compliant voltage contacts 316 are made in a spiral configuration which can achieve a high amount of compliance within a small area.

Referring again to FIG. 4, the voltage contacts 120 of the MPA 103 align with the compliant voltage contacts 316 in the pivot platform 304 of the pivot mount 300. Once the MPA is clamped onto the pivot mount pivot platform, for example, using an electrostatic clamp contact on the pivot mount platform, the compliant voltage contacts 316 exert a pressure upon the MPA voltage contacts 120 to achieve low contact resistance connections.

FIGS. 6A-7 illustrate various structural aspects of a pivot mount 300. In an embodiment pivot mount 300 includes a base 302, a pivot platform 304, a plurality of primary spring arms 306, and a plurality of secondary spring arms 307. Referring to FIG. 6A, in an embodiment each primary spring arm 306 is fixed to the pivot platform 304 at a corresponding inner root 350, and fixed to the base 302 at a corresponding outer root 352. Each primary spring arm 306 includes at least one switch-back along an axial length 354 of the primary spring arm such that a pair of lengths of the primary spring arm adjacent the switch-back are parallel to each other. In the embodiment illustrated in FIG. 6A, each primary spring arm 306 includes an inner switch-back 356 along an inner length of the spring arm and an outer switch-back 358 along an outer length of the spring arm. In an embodiment, an inner length 370 of the spring arm extending from the pivot platform 304 (along the axial length 354 of the spring arm 306) is perpendicular to the inner root 350. In an embodiment, an outer length 372 of the spring arm extending from the base 302 (along the axial length 354 of the spring arm 306) is perpendicular to the outer root 352.

Referring now to FIG. 6B, in an embodiment each secondary spring arm 307 is fixed to the pivot platform 304 at a corresponding inner root 351, and fixed to the base 302 at a corresponding outer root 353. Each secondary spring arm 307 includes at least one switch-back along an axial length 355 of the secondary spring arm such that a pair of lengths of the secondary spring arm adjacent the switch-back are parallel to each other.

FIGS. 6C-6D are close-up top view illustrations of Detail B in FIG. 6B in accordance with embodiments. In the embodiment illustrated in FIG. 6C, each secondary spring arm 307 includes an inner switch-back 357 along an inner length of the secondary spring arm and an outer switch-back 359 along an outer length of the secondary spring arm. Each secondary spring arm 307 may additionally include one or more intermediate switch-backs 349A, 349B along a length of the secondary spring arm between the inner and outer switch-backs 357, 359. As illustrated in FIG. 6C, an inner length 371 of the secondary spring arm extending from the pivot platform 304 (along the axial length 355 of the secondary spring arm 307) is perpendicular to the inner root 351. In an embodiment, an outer length 373 of the spring arm extending from the base 302 (along the axial length 355 of the secondary spring arm 307) is perpendicular to the outer root 353. In the embodiments illustrated, each switch-back along the axial length of the primary spring arm and secondary spring arm results in a parallel pair of lengths of the spring arm adjacent the switch-back.

In the embodiment illustrated in FIG. 6D, a portion of the secondary spring arm immediately adjacent the intermediate switch-back 349A includes a first length 361A and a second length 363A of the secondary spring arm that are parallel to each other. A portion of the secondary spring arm immediately adjacent the intermediate switch-back 349B includes a first length 361B and a second length 363B of the secondary spring arm that are parallel to each other. Similarly, a portion of the secondary spring arm immediately adjacent the inner root 351 includes a first length 365 and a portion of the secondary spring arm immediately adjacent the outer root 353 includes a second length 367 of the secondary spring arm that are parallel to each other. Referring briefly to FIG. 14, in an embodiment each primary spring arm and secondary spring arm is characterized as including a series of spring arm segments with endpoints characterized by a boundary condition of slope θ equal to zero. As described in further detail with regard to FIGS. 15A-15B, the endpoints of the spring arm segments correspond to local maxima or local minima bending moments, where the slope of the deformed spring arm segment changes in sign between positive and negative for a given deflection of the pivot platform. In the particular embodiments illustrated in FIG. 6D and FIG. 14 length 365 is within secondary spring arm segment 1, length 361A is within secondary spring arm segment 2, lengths 363A, 363B are within secondary spring arm segment 3, length 361B is within secondary spring arm segment 4, length 367 is within secondary spring arm segment 5. In the particular embodiment illustrated, the secondary spring arm 307 is characterized by a right angle omega (Ω) shape, with each secondary spring arm segment having an axial length parallel to each other, and secondary spring arm segment 3 being longer than secondary spring arm segments 1, 2, 4, 5.

In accordance with embodiments, strain sensing elements may be located along the lengths of the secondary spring arm adjacent switch-backs or roots. Furthermore, reference gages may be located adjacent the strain sensing elements. FIG. 6E is a close-up top view illustration of Detail D in FIG. 6D in accordance with an embodiment. In the particular embodiment illustrated in FIG. 6E, a first strain sensing element 320 is located at the first length of 361B the secondary spring arm adjacent the intermediate switch-back 349B, and a second strain sensing element 320 is located at the second length 363B of the secondary spring arm adjacent the intermediate switch-back 349B. Furthermore, first reference gage 340 is located adjacent the first strain sensing element 320 at the first length 361B, and a second reference gage 340 is located adjacent the second strain sensing element 320 at the second length 363B. Referring again to FIG. 6D, reference gages 340 are also located adjacent the strain sensing elements 320 at the lengths 365, 367 located near and perpendicular to the inner and outer roots 351, 353 and at lengths 361A, 363A adjacent intermediate switch-back 349A. As illustrated, the strain sensing elements 320 and reference gages 340 are located a certain distance away from the roots and switch-backs to avoid stray strain regions, where some stress concentrations may be present due to patterning the substrate. Further detail regarding placement of the strain sensing elements 320 and reference gages is provided in the following description with regard to FIGS. 14-16.

Referring again to FIG. 6E, in the particular embodiment illustrated the strain sensing elements 320 and reference gages 340 along the first and second lengths 361B, 363B of the secondary spring arm adjacent an intermediate switch-back 349B are shown in more detail. In an embodiment, strain sensing elements 320 may be strain gages that measure deformation of secondary spring arm 307. The strain gages may exhibit an electrical resistance that varies with material deformation. More specifically, the strain gages may deform when secondary spring arm 307 deforms. That is, the strain gage design may be selected based on environmental and operating conditions associated with the transfer of micro devices from a carrier substrate, to achieve the necessary accuracy, stability, cyclic endurance, etc. Accordingly, the strain gages may be formed from various materials and integrated with the spring arm in numerous ways to achieve this goal. Several such embodiments are described below.

A strain gage may be separately formed from secondary spring arm 307 and attached thereto. In an embodiment, the strain gage includes an insulative flexible backing that supports a foil formed from polysilicon and electrically insulates the foil from secondary spring arm 307. The foil may be arranged in a serpentine pattern, for example. An example of an attachable strain gage is a Series 015DJ general purpose strain gage manufactured by Vishay Precision Group headquartered in Malvern, Pa. A strain gage that is separately formed from secondary spring arm 307 may be attached to secondary spring arm 307 using numerous processes. For example, the strain gage backing may be directly attached to secondary spring arm 307 with an adhesive or other bonding operation. More specifically, strain gage backing may be fixed to a surface of secondary spring arm 307 using solder, epoxy, or a combination of solder and a high-temperature epoxy.

In another embodiment, a strain gage may be formed on secondary spring arm 307 in a desired pattern, such as a serpentine pattern. In an embodiment, a strain gage may be formed directly on secondary spring arm 307 using a deposition process. For example, constantan copper-nickel traces may be sputtered directly on secondary spring arm 307 in a serpentine pattern. The dimensions of a strand of a sputtered strain gage having a serpentine pattern may be about 8 micron width with about an 8 micron distance between strand lengths and may be deposited to a thickness of about 105 nanometers.

In another embodiment, the material of secondary spring arm 307 may be modified to form an integrated strain gage. More specifically, secondary spring arm 307 may be doped so that the doped region of the spring arm exhibits piezoresistive behavior. As an example, the surface of secondary spring arm 307 may be doped silicon. The doped material may be in a serpentine pattern, having dimensions that vary with an applied strain. Thus, the strain gage may be fully integrated and physically indistinct from the remainder of secondary spring arm 307.

During the transfer of micro devices from a carrier substrate, secondary spring arm 307 and strain sensing elements 320 may be subjected to elevated temperatures, and thus, temperature compensation may be necessary. In an embodiment, strain sensing element 320 (strain gage) may be self-temperature compensated. More specifically, strain gage material may be chosen to limit temperature-induced apparent strain over the operating conditions of the transfer process. However, in an alternative embodiment, other manners for temperature compensation may be used. For example, temperature compensation may be achieved using a reference gage technique.

In an embodiment, strain sensing element 320 may be a strain gage on secondary spring arm 307 having a pattern (e.g. serpentine) of lengthwise strands that align in a direction of anticipated normal strain at the surface of the spring arm. Referring to FIGS. 6D-6E, in an embodiment, a reference gage technique utilizes a reference gage 340 to compensate for strain sensing element 320. More particularly, reference gage 340 may be located adjacent strain sensing element 320 in the same area of strain. While strands of strain sensing element 320 may align with the direction of applied strain, strands of reference gage 340 may extend perpendicular to the strands of strain sensing element 320 and to the direction of applied strain. Alternatively, reference gage 340 may be located in a non-strain area of the pivot mount 300, apart from strain sensing element 320, which is located in a high strain area of secondary spring arm 307. For example, reference gage 340 may be located on base 302 or pivot platform 304. In each configuration, strain sensing element 320 detects a strain applied to secondary spring arm 307 and reference gage 340 detects strain from thermal effects on the pivot mount 300. Accordingly, a comparison of strain in the strain sensing element 320 and reference gage 340 may be used to determine, and compensate for, strain related to thermal expansion of secondary spring arm 307.

In particular, the strands 341 in the references gages 340 are oriented perpendicular to strands 321 in the strain sensing elements 320. As will become more apparent in the following description, the normal strain at the surface that results at the first and second lengths 361B, 363B of the secondary spring arm during operation of the pivot mount is substantially parallel to the strands 321 in the strain sensing elements, and perpendicular to the strands 341 in the reference strain gages. Similar strain relationships are found at the other described locations (e.g. 365, 361A, 363A, 367) for strain sensing elements where normal strain at the surface that occurs during operation of the pivot mount is substantially parallel to the strands in the strain sensing elements 320.

In accordance with embodiments, the one or more secondary spring arms 307 are characterized as having a lower stiffness than the one or more primary spring arms 306. As such, a greater amount of strain can be measured in the secondary spring arms than the primary spring arms, resulting in strain signal amplification. The relative stiffness of the primary spring arms and secondary spring arms may be selected by adjusting the length, width, or thickness of the spring arms or materials of the spring arms. For example, referring to FIGS. 6A-6B the primary axis length 354 may be greater than the secondary axis length 355. An average width along the primary axis length 354 may be wider than an average width along the secondary axis length 355. The primary spring arm 306 and secondary spring arm 307 may also share the same average thickness along their respective axial lengths. In an embodiment, the primary spring arm and the secondary spring arm are formed of the same material. For example, each may be formed from the same silicon substrate, and each may be integrally formed.

In an embodiment, the secondary spring arm 307 has a lower average thickness along its axial length 355 than the primary spring arm 306 has along its axial length 354. In this manner the relative stiffness can be selected by modulating thickness of the spring arms. FIGS. 6F-6H are cross-sectional side view illustrations along section A-A in FIGS. 6A-6B of a method of modulating stiffness by reducing layer thickness in accordance with an embodiment. Referring to FIG. 6F, the process may start with a patterned base substrate 301 such as a silicon substrate including the base 302, pivot platform 304, primary spring arms 306 and secondary spring arms 307. The relative thicknesses may then be modulated by selectively etching away a thickness of the secondary spring arm 307 as illustrated in FIG. 6G, resulting in the pivot platform configuration shown in FIG. 6H in which the primary spring arms 306 are thicker than the secondary spring arms 307.

In an embodiment, one or more additional layers with differentiated features are built up along the primary spring arms 306 compared to the secondary spring arms 307. FIGS. 6I-6J are cross-sectional side view illustrations along section A-A in FIGS. 6A-6B of a method of modulating stiffness with layer build up in accordance with an embodiment. Referring to FIG. 6I, the process may start with a patterned base substrate 301 such as a silicon substrate including the base 302, pivot platform 304, primary spring arms 306, and secondary spring arms 307 of a first thickness. A stiffener layer 601 may then be formed on the base substrate 301 as illustrated in FIGS. 6I-6J. In an embodiment, the stiffener layer 601 is bonded to the base substrate 301, for example by wafer bonding. The stiffener layer 601 may be formed of the same material or a different material than the base substrate 301 in order to achieve the desired stiffness. Likewise, thickness of the stiffener layer 601 may be modulated. As shown, the stiffener layer 601 may be patterned similarly as the base substrate 301, including the base 302, pivot platform 304, and primary spring arms 306 of a second thickness. In an embodiment, a gap or a reduced thickness of the stiffener layer 601 exists where the secondary spring arms 307 exist in the base substrate 301 in order to modulate the thickness. In the embodiment illustrated, a composite pivot mount structure is formed by bonding the base substrate 301 and stiffener layer 601. The resultant structure includes primary spring arms 306 with a total thickness including the sum of the first and second thicknesses, with the secondary spring arms having only the first thickness.

FIG. 7 is a top view illustration of a pivot mount including electrical routing in accordance with an embodiment. As illustrated, wiring can be routed on the top surface of the pivot mount for operation of various components. In an embodiment wiring 380 is provided for operation of the strain sensing elements 320 and reference gages 340. In an embodiment wiring 382 is provided for operation of the electrostatic clamp contacts 318. In an embodiment wiring 384 is provided for operation of the compliant voltage contacts 316. In the particular embodiment illustrated, the wiring 384 connects with the electrodes 317 for the compliant voltage contacts 316, where the electrodes form a spiral pattern within the spiral channels 310 forming the compliant voltage contacts. Wiring 380, 382, and 384 can run over one or more portions of the pivot mount including the base 302, primary spring arms 306, secondary spring arms 307, and pivot platform 304. Wiring 380, 382, and 384 may be formed using a suitable technique such s sputtering or e-beam evaporation, or may be a wire that is bonded to the pivot mount.

Wiring 380, 382, and 384 may be routed to an electrical connection, such as a flex circuit 308, at an edge of the base 302 of the pivot mount. For example, an operating voltage can be applied trough the flex circuit 308 to operate the electrostatic clamp contacts 318 to clamp the MPA onto the pivot mount 300. Another operating voltage can be applied through the flex circuit 308 to operate the compliant voltage contacts 316 which transfer an operational voltage to the array of electrostatic transfer heads in order to provide a grip pressure to pick up micro devices. Additionally, the flex circuit 308 can transfer the feedback signals from the strain sensing elements 320 and reference gages 340 to a position sensing module or computer 108 to regulate actuation and spatial orientation of the transfer head assembly 200.

Referring now to FIG. 8A, strain at any point in a body may be described by nine strain components. These include three normal strains (εx, εy, εz) and six shear strain components (εxy, εxz, εyx, εyz, εzx, and εzy). Strain components in a thin structure are illustrated in FIG. 8B. For a thin pivot mount structure shear strains on the surface (εzx and εzy) and out-of-plane normal strain (εz) are not significant. This idealization is known as plane stress. Accordingly, in an embodiment the strain gages (strain sensing elements and reference gages) on the surface of the pivot mount will measure components of the normal strains εx and εy. In an embodiment, the pivot mount includes regions of strain loaded only in either pure εx or pure εy and directs substantially all available strain into measurable strain.

Referring now to FIGS. 9-10, the idealization of plane stress is illustrated as realized in accordance with embodiments. FIG. 9 is an illustration of a spring arm under pure bending in accordance with an embodiment. In such an embodiment, the spring arm undergoing pure bending may have a single normal strain component aligned with the spring arm axial length. A reference gage 340 may be oriented perpendicular to the spring arm axial length and not measure any strain due to bending. FIG. 10 is an illustration of a spring arm in both bending and torsion. In such a configuration, normal strain components and shear strain components are produced in multiple directions. In this case both the strain gage 320 and reference gage 340 may measure non-zero strain, which may reduce the ability of the reference gage 340 to compensate for temperature changes.

In order to illustrate strain confinement within the pivot mount, a pivot mount with a uniform z displacement of the pivot platform 304 is illustrated in FIGS. 11A-11E along with modeling data for strain fields located within the pivot mount. Referring to FIG. 11A, a pivot platform 304 of pivot mount 300 is deflected with a uniform z displacement. Such deflection may be typical during a normal pick and place operation with the mass transfer tool, though the amount of deformation illustrated in FIG. 11A is exaggerated for illustrational purposes. In the particular embodiment illustrated in FIG. 11A, the primary spring arm 306 along the first length 366 of the spring arm adjacent the outer switch-back 358 and the first length 360 of the spring arm adjacent the inner switch-back 356 have a negative curvature and are in a condition of negative (compressive) normal strain at the surface. In the particular embodiment illustrated in FIG. 11A, the primary spring arm 306 along the second length 364 the spring arm adjacent the outer switch-back 358 and the second length 362 of the spring arm adjacent the inner switch-back 356 have a positive curvature and are in a condition of positive (tensile) normal strain at the surface.

Referring to FIG. 11B, a pivot platform 304 of pivot mount 300 is deflected with a uniform z displacement as described with regard to FIG. 11A. As illustrated in FIG. 11B, the secondary spring arm 307 includes lengths 361A, 363B, 367 that are in condition of negative (compressive) normal strain at the surface, and lengths 365, 363A, 361B that are in a condition of positive (tensile) normal strain at the surface.

In accordance with embodiments, a pivot mount structure achieves a high strain sensing sensitivity and generates a feedback signal with a high signal to noise ratio by locating strain sensing elements on secondary spring arms that have a lower stiffness than the primary spring arms. In this manner secondary spring arms undergo more strain for a given pivot platform displacement than the primary spring arms and a higher strain signal is produced. The strain sensing sensitivity and feedback signal may be further increased by locating strain sensing elements at locations of the secondary spring arms where equal and opposite strain responses are measured, such as at inner and outer roots and/or on opposite sides of switch-backs. In this manner, strain signal for a given platform displacement may be effectively doubled, while also reducing noise for a given strain signal since the differential sensing can be used to effectively cancel the noise.

Referring to FIG. 11C, modeling data is provided for the z displacement illustrated in FIGS. 11A-11B illustrating normal strain at the outer surface of the pivot mount in the x direction, εx. As shown, when in the condition of uniform z displacement, the high εx strain regions are located along lengths of the secondary spring arms extending in the x-direction between the base and pivot platform, while minimal or no εx strain is located along the secondary spring arms extending in the y-direction between the base and pivot platform. In addition, the high strain regions are concentrated in the secondary spring arms rather than the primary spring arms. Specifically, regions in a condition of positive (tensile) normal strain at the surface with the highest εx are located at lengths 365, 363A, 361B, and regions in a condition of negative (compressive) normal strain at the surface with the highest εx are located at lengths 361A, 363B, 367.

In the embodiment illustrated in FIG. 11C, lengths 361A, 363A adjacent the switch-back 349A have equal and opposite normal strains, and lengths 361B, 363B adjacent the switch-back 349B have equal and opposite normal strains. In an embodiment, length 365 adjacent the inner root 351 and length 367 adjacent outer root 353 have equal and opposite normal strains.

In an embodiment a pair of adjacent secondary spring arms is located between a pair of primary spring arms. For example, the pair of adjacent secondary spring arms may be mirror images of each other, providing additional redundancy of the strain gauges. In an embodiment, strain at length 361A is the same in pair of adjacent mirror image secondary spring arms. Lengths 363B, 367, 365, 363A, 361B in the pair of adjacent mirror image secondary spring arms may also have the same corresponding strains.

In an embodiment, a pair of opposite secondary spring arms is located on opposite sides of the pivot platform. For example, the pair of opposite secondary spring arms may be mirror images of each other, providing additional redundancy of the strain gauges. In an embodiment, strain at length 361A is the same in pair of opposite mirror image secondary spring arms. Lengths 363B, 367, 365, 363A, 361B in the pair of opposite mirror image secondary spring arms may also have the same corresponding strains.

Referring to FIG. 11D, modeling data is provided for the z displacement illustrated in FIGS. 11A-11B illustrating normal strain at the outer surface of the pivot mount in the y direction, εy. As shown, when in the condition of uniform z displacement, the high εy strain regions are located along lengths of the secondary spring arms extending in the y-direction between the base and pivot platform, while minimal or no εy strain is located along the secondary spring arms extending in the x-direction between the base and pivot platform. In addition, the high strain regions are concentrated in the secondary spring arms rather than the primary spring arms. Specifically, regions in a condition of positive (tensile) normal strain at the surface with the highest εy are located at lengths 365, 363A, 361B, and regions in a condition of negative (compressive) normal strain at the surface with the highest εx are located at lengths 361A, 363B, 367. Thus, the highest εy regions illustrated in FIG. 11D are similar to the highest εx regions illustrated in FIG. 11C, rotated 90 degrees. Furthermore, the description of equal and opposite normal strains, and mirror images of the spring arms is the same in FIG. 11D, rotated 90 degrees.

FIG. 11E is an illustration of modeling data for equivalent strain magnitude at the outer surface of the pivot mount in both εx and εy for the z displacement illustrated in FIGS. 11A-11B. As shown, substantially equal strain magnitudes are measured at each of the secondary spring arms. FIG. 11F is an illustration of modeling data for shear strain at the outer surface of the pivot mount for the z displacement illustrated in FIGS. 11A-11B. As illustrated, there is substantially no measurable shear strain at the surface. Thus, the modeling data provided in FIGS. 11C-11E illustrates a pivot mount configuration with substantially uniform bending moments in the high strain regions of the spring arms, in which strain is substantially parallel to axial lengths of the spring arms.

FIG. 12 is a schematic illustration of an idealized beam of length L with one end fixed and subject to simultaneous transversely applied force F and bending moment M_(L) at the free end and with zero slope boundary conditions at both ends. In the case shown, the boundary conditions at both ends are such that the slope θ of the deformed beam is equal to zero. The beam and loading shown in FIG. 12 is in essence a simplification and idealization of the individual spring arm segments comprising the primary spring arms and secondary spring arms in the pivot mount when under load. While the pivot mount spring arm structures in the described embodiments include switch-backs, the underlying behavior of each beam segment in the secondary spring arms can be represented in simplified form by the idealized beam shown in FIG. 12. Both the primary and secondary spring arms in the pivot mount can each be thought of as two or more of such idealized beams arranged in series with switch-backs forming the union between each serial beam segment. Further, the ends of the idealized beam in FIG. 12 are similarly analogous to the inner and outer roots of the spring arms.

It can be shown that the moment M_(L) applied at a point x=L at the end of the beam shown in FIG. 12 that meets the boundary conditions of zero slope at each end is, in terms of the applied force, F:

$\begin{matrix} {M_{L} = \frac{FL}{2}} & (1) \end{matrix}$ Further, it can be shown that the displacement δ of the beam of length L shown in FIG. 12 at a point x=L is given by:

$\begin{matrix} {\delta = \frac{{FL}^{3}}{12\;{EI}}} & (2) \end{matrix}$ where E is the Young's Modulus of the beam and I is the area moment of inertia about the neutral axis. The bending stress σ in a beam is given by:

$\begin{matrix} {\sigma = \frac{My}{I}} & (3) \end{matrix}$ where M is the bending moment at a point along the length of the beam and y is the distance from the neutral plane. This equation shows that this stress will be maximum at a point y=c, where c is the distance from the neutral plane to the outer surface of the beam.

Referring to FIG. 13, the shear force and bending moment diagrams for the idealized beam depicted in FIG. 12 is shown. Such diagrams are derived using elementary mechanics of materials methods. Referring to the bending moment diagram, it is evident that the bending moment is equal but of opposite sign at each end of the beam, and further, that the absolute value of the bending moment is maximum at the ends of the beam. Because the value of bending moment M is maximum and the ends of the beam, the stress σ will also be maximum at the ends of the beam shown in FIG. 12, or when M=M_(L). Regions of the beam in which the bending moment has a positive value will have positive or tensile stress. Correspondingly, regions of the beam in which the bending moment has a negative value will have a negative or compressive stress. The sign of the shear force and bending moment is dependent on the sense of the load applied to the beam. If the sense of the applied load is reversed, the sign of the shear force and bending moment is reversed while the absolute values of the shear force and bending moment will remain unchanged. Thus, if the sense of the applied load is reversed the sign of the resulting stress will also reverse. Stress can be related to strain c through Hooke's Law: σ=Eε  (4) Combining equations 3 and 4, strain at the outer surface of the beam can be expressed as:

$\begin{matrix} {\varepsilon = \frac{Mc}{EI}} & (5) \end{matrix}$ Rearranging equation 2 and inserting the result into equation 1:

$\begin{matrix} {F = \frac{12{EI}\;\delta}{L^{3}}} & (6) \\ {M_{L} = \frac{6{EI}\;\delta}{L^{2}}} & (7) \end{matrix}$ Inserting the expression for the bending moment at the end of the beam in equation 7 into equation 5 it can be shown that the normal strain c at the surface of the beam at a position x=L for a given loading will be:

$\begin{matrix} {\varepsilon = \frac{6\; c\;\delta}{L^{2}}} & (8) \end{matrix}$ Recognizing that the distance from the neutral axis c to the surface of the spring arm and deflection δ will be identical for both the primary and secondary spring arms, the above expression for strain can be expressed as the following proportionality, in which the resulting strain c is inversely proportional to the length L of the beam squared:

$\begin{matrix} {\varepsilon \propto \frac{1}{L^{2}}} & (9) \end{matrix}$

Hence, an ideal strain amplifying secondary spring arm structure will have a total length L that is substantially shorter than the primary spring arms of the pivot mount. For example, if the secondary spring arms of the pivot mount are half the composite length of the primary spring arms, the strain resulting from a given loading in the secondary spring arm will be four times the strain in the primary spring arms. Thus, a strain gage placed on a secondary spring arm having a total length half the length of a primary spring arm may produce a signal four times as a large as a strain gage placed at a corresponding position on the primary spring arm for the same displacement of the pivot mount platform. Stated another way, the sensitivity of a pivot mount as a displacement sensing device may improve fourfold by incorporating such structures.

Referring to FIG. 14, a detail view of a strain amplifying secondary spring arm structure with each beam segment labeled sequentially starting at the inner root to the outer root is shown in accordance with an embodiment. Further, the endpoints of each beam segment are identified. For example, spring arm beam segment 1 has corresponding endpoints 1 a and 1 b. Additionally, it will be apparent that endpoint 1 a corresponds to the inner root of the secondary spring arm and that endpoint 5 b corresponds to the outer root of the secondary spring arm.

Referring to FIG. 15A, the bending moment diagram along the length of the strain amplifying secondary spring arm structure depicted in FIG. 14 resulting from a load applied to the pivot mount platform is shown in accordance with an embodiment. As with the diagrams shown in FIG. 13, this diagram is derived using elementary mechanics of materials methods. The bending moment diagram shown has been divided into regions corresponding to the individual beam segments defined for the secondary spring arm structure shown in FIG. 14. Referring to the diagram, the absolute value of the bending moment is maximum at end points 1 a, 2 b, 3 a, 3 b, 4 a, and 5 b. By extension, the absolute value of strain is also a maximum at end points 1 a, 2 b, 3 a, 3 b, 4 a, and 5 b.

The sign of the shear force and bending moment is dependent on the sense of the load applied to the beam. If the sense of the applied load is reversed, the sign of the shear force and bending moment is reversed while the absolute values of the shear force and bending moment will remain unchanged. Thus, if the sense of the applied load is reversed the sign of the resulting stress and, it follows, strain will also reverse. Referring to FIG. 15B, the bending moment diagram for the secondary spring arm of FIG. 14 for the case of equal applied load with opposite sense to that depicted in FIG. 15A is shown in accordance with an embodiment. Note that the bending moment for both load cases is equal among the grouping of endpoints 1 a, 3 b, and 4 a and that the bending moment is also equal among the grouping of endpoints 2 b, 3 a, and 5 b. Further, note that the magnitude of the bending moment and, hence, strain response is always equal but of opposite sign between the grouping of endpoints 1 a, 3 b, and 4 a and the grouping of endpoints 2 b, 3 a, and 5 b. It is possible to form “correlated pairings” or, generally, “correlated groupings” of strain gages by combining the signals from stain gages that are subjected to a correlated strain response.

In accordance with embodiments, some amount of localized strains are found at various locations within the pivot mount due to local stress concentrations, however these do not affect the strain measurement because the strain gages are located away from the localized strain regions. For example, local stress concentrations may be found at the ends of channels 310 defining the switch-backs or roots near end points 1 a, 2 b, 3 a, 3 b, 4 a, and 5 b. Accordingly, while strain may reach a theoretical maximum at the segment endpoints, in an embodiment strain gages and reference gages are located a specific distance away from the end points 1 a, 2 b, 3 a, 3 b, 4 a, and 5 b so that they are not located at fringe strain regions associated with local stress concentrations yet still be located at or near the regions of highest strain.

Strain sensing elements 320 and reference gages 340 may be arranged into sensors so that the resulting sensor signals are correlated. A set of sensors is considered correlated, or dependent, if the signal of a missing or broken gage in the sensor may be approximated from the remaining set of signals. A minimum set of independent strain signals equal to the number of desired position measurements is required to calculate those measurements. Correlated strain signals in excess of the minimum required set may be included in the position calculation and used to improve the signal to noise ration of the measurement. If a strain gage (320, 340) or sensor failure occurs the calculation may be adjusted to maintain position output albeit with a reduced signal to noise ratio. In this way correlated signals provide redundancy as well as an improved signal to noise ratio.

Referring to FIG. 16, a pivot mount including 24 correlated strain sensors is illustrated in accordance with an embodiment. Specifically, FIG. 16 is an exemplary illustration similar to FIG. 7 described above, including 48 total strain sensing elements 320 (strain gages) and 48 total reference gages 340. In such a configuration, a pair of strain sensing elements (strain gages) and references gauges on opposite sides of a switch-back or at the inner/outer roots may correspond to a single strain sensor. As previously described, these pairs of strain sensing elements 320 on opposite sides of a switch-back or at the inner/outer roots measure opposite strain types, of equal magnitude. Accordingly, these pairs of strain gages (as well as the corresponding reference gages 340) can also be considered strain sensors, which may be correlated with other strain sensors. The strain sensors illustrated in FIG. 16 may be linearly dependent sets (correlated pairs) depending upon whether the pivot platform is rotated about the x-axis, rotated about the y-axis, or is subjected to a vertical displacement. Table I below describes certain correlated pairs of the exemplary embodiment.

TABLE I Correlated pair strain sensors Under rotation about the y-axis signal 1 = −signals 7, 10 signal 2 = −signals 9, 12 signal 3 = −signals 8, 11 signal 1 = signal 4 signal 2 = signal 5 signal 3 = signal 6 Under rotation about the x axis signal 13 = −signals 19, 22 signal 14 = −signals 21, 24 signal 15 = −signals 20, 23 signal 13 = signal 16 signal 14 = signal 17 signal 15 = signal 18 Under vertical displacement signal 1 = signals 4, 7, 10, 13, 16, 19, 22 signal 2 = signals 5, 9, 12, 15, 18, 20, 23 signal 3 = signals 6, 8, 11, 14, 17, 21, 24

In the above exemplary embodiment, several correlated pairs are described for a 24 channel (signal) operation, with each channel corresponding to a signal produced by a pair of strain gages and references gages. Under normal operation, the feedback signal produced by the exemplary pivot mount operating under normal operation can be converted into a synthesized output signal by a transformation matrix. A generalized transformation matrix for converting a pivot mount feedback signal to a synthesized output signal is represented in equation (10) for n strain signal inputs to 3 position measurement outputs (e.g. tilt, tip, z):

$\begin{matrix} {\begin{bmatrix} {Out}_{1} \\ {Out}_{2} \\ {Out}_{3} \end{bmatrix} = {\begin{bmatrix} A_{1} & A_{2} & A_{3} & \ldots & A_{n} \\ B_{1} & B_{2} & B_{3} & \ldots & B_{n} \\ C_{1} & C_{2} & C_{3} & \ldots & C_{n} \end{bmatrix}\begin{bmatrix} S_{1} \\ S_{2} \\ S_{3} \\ \ldots \\ S_{n} \end{bmatrix}}} & (10) \end{matrix}$

While embodiments of pivot mounts have been described thus far in a square configuration, with secondary spring arms extending along the x-direction or y-direction, embodiments are not so limited. Indeed, the strain sensing elements and reference gages can be located along a number of directions. In an embodiment illustrated in FIG. 17, a pivot mount 300 includes a base 302, pivot platform 304, primary spring arms 306 and secondary spring arms 307. Each secondary spring arm 307 is fixed to the pivot platform 304 at a corresponding inner root 351, and fixed to the base 302 at a corresponding outer root 353 as previously described. The pivot mount illustrated in FIG. 17 differs from other embodiments of pivot mounts described herein in that the secondary spring arms 306, 307 are arranged in a generally equilateral triangular configuration, rather than a generally square configuration. As a result, strain measured is not located within the only the εx and εy directions. Nevertheless, the same results of equal and opposite strain, uniform bending moments in the high strain regions, and distributed, correlated pairs is achieved. Accordingly, while embodiments have been described specific to creating and measuring strain in the εx and εy directions, embodiments are not so limited, and pivot mount feedback signals may be converted into a synthesized output signal for a variety of geometries.

In accordance with embodiments, the transfer head assembly 200 may adjust the orientation of the MPA 103 until a desired amount of and/or a desired distribution of pressure across pivot mount 300 is sensed by the pivot mount 300 strain sensing elements 320. Thus, the transfer head array 115 on MPA 103 may be actively aligned with an array of micro devices on a mating substrate. For example, the spatial orientation representing alignment may be predetermined to include a plane passing through the transfer head array 115 being parallel to a plane passing through the array of micro devices. Alternatively, the spatial orientation representing alignment may include the planes not being parallel, but rather, being in some predetermined mutual orientation, such as angled such that only a portion of the transfer head array 115 make contact with respective micro devices when the arrays are brought together. More particularly, the spatial orientation representing alignment of the transfer head array 115 with the array of micro devices may be any predetermined spatial orientation. Such spatial orientation may be monitored, sensed, and measured to determine system characteristics such as distribution of pressure across pivot mount 300. Thus, the measured system characteristics may be used as a proxy to represent alignment. Active alignment may increase the transfer rate of micro devices, since fine-alignment may be accomplished while picking up, and similarly while releasing, the micro devices. Furthermore, active alignment may be made on-the-fly without parasitic translation of the transfer head array 115 that may otherwise smear and damage the array of micro devices. Such on-the-fly adjustments may be useful when a donor substrate, e.g., carrier substrate, and/or a display substrate, e.g., receiving substrate, include surface irregularities and non-planar contours.

Referring to FIG. 18A, a schematic illustration of a control scheme for regulating a transfer head assembly is shown in accordance with an embodiment. More particularly, the control loop may include multiple sub-loops that process a combination of position and strain inputs. The actuators of transfer head assembly may be driven by the sub-loops, first toward an initial desired location, and if contact between MPA 103 and a target substrate is sensed, then the initial desired location may be modified to move MPA 103 toward a desired stress state, e.g., to evenly distribute pressure across MPA 103 and/or to achieve a desired level of pressure at one or more locations on pivot mount 300 based on a deflection of the pivot mount 300 secondary spring arms 307.

A primary input 1802 may define a set of reference signals that correspond to an initial desired state of MPA 103. More specifically, primary input 1802 may define a target spatial location of MPA 103 relative to an anticipated location of a micro device array or substrate surface. Primary input 1802 may be fed into one of several inner loops, each of which may correspond to an individual actuator. For example, x-actuator inner loop 1804 may correspond to a control loop for controlling an x-actuator of the transfer head assembly, and thus MPA 103, to tip about a remote rotational center. Similarly, y-actuator inner loop 1806 may correspond to a control loop for controlling a y-actuator of the transfer head assembly, and thus MPA 103, to tilt about the remote rotational center. Also, z-actuator inner loop 1808 may correspond to a control loop for controlling a z-actuator of the transfer head assembly and thus a location of MPA 103 along a z-axis. Therefore, the combination of inner loops allow for the control of actuators that adjust a tip, tilt, and z-spatial orientation of MPA 103.

In an embodiment, inner loop control of transfer head assembly 200 actuators results in a primary output 1810. More specifically, primary output 1810 may be an instantaneous geometric configuration of transfer head assembly 200 resulting from actuator movement. The geometric configuration may be inferred from data supplied by encoders or other sensors that track spatial position of individual transfer head assembly 200 components. That is, the geometric configuration may include a combination of individual geometric configurations such as a tip position, tilt position, and z-position. Primary output 1810 may also relate to a spatial position of MPA 103 as inferred from known physical dimensions of transfer head assembly 200 components. Alternatively, MPA 103 surface location may be sensed directly using, e.g., laser micrometers, accelerometers, etc., to provide spatial orientation feedback that may be included directly in primary output 1810. Thus, a position of MPA 103 may be inferred or sensed to determine whether primary output 1810 has been achieved, i.e., equals the intended primary input 1802. However, although MPA 103 may be driven toward a target substrate to achieve the positional command of primary input 1802, in some cases, MPA 103 may contact the target substrate. Furthermore, once contact is detected, primary input 1802 may be modified by additional commands from several actuator outer loops, to achieve a neutral tip and tilt deformation of pivot mount 300 with a desired pressure distribution across pivot mount 300. Accordingly, MPA array 103 may be driven to a tip deflection, tilt deflection, and z-compression target within an accuracy in the submicron range, e.g., on the order of less than about 250 nm.

After contact between a transfer head array 115 of MPA 103 and a micro device has been made, MPA 103 may be finely adjusted based on pressure feedback from the pivot mount 300. More particularly, fine adjustment of MPA 103 may be enabled in response to system recognition of a contact disturbance 1812. In an embodiment, enable logic is included to determine whether a contact disturbance 1812 is sensed prior to MPA 103 achieving the desired primary input 1802, and if a contact disturbance 1812 is sensed, additional control loops may be closed to permit fine adjustment of the transfer head assembly 200. More specifically, additional control loops may be closed to drive MPA 103 toward tip deflection, tilt deflection, and z-compression targets, rather than toward the initial positional target of primary input 1802.

In an embodiment, a contact disturbance 1812 is sensed when, e.g., MPA 103 contacts a mating substrate out of alignment. For example, if MPA 103 and the mating substrate make contact in perfect alignment, the primary output 1810 may equal the primary input 1802 and micro devices may then be gripped by transfer head array 115 without requiring additional adjustment. However, if MPA 103 and the mating substrate are not perfectly aligned, displacement or strain measurements from each strain sensing element 320 on pivot mount 300 may be substantially different from each other and/or the desired level of pressure may not be achieved. That is, in an embodiment, an expected or desired tip, tilt, and compression state must be satisfied prior to initiating electrostatic gripping. If the desired state is not achieved, displacement or strain measurements may be fed as feedback signals 1814.

In an embodiment, feedback signals 1814 correspond to analog signals from the strain sensing elements 320 and references gages 340. In the exemplary embodiment above, feedback signals 1814 may include twenty four sensor signals from forty eight separate strain sensing elements 320 and forty eight reference gages 340. The feedback signals 1814 may be conditioned by a signal conditioning and combination logic 1815 to transform the analog signals into a synthesized output signal representing a strain state of a respective strain sensing element. These synthesized output signals may furthermore be combined by signal conditioning and combination logic 1815 to synthesize one or more of a pivot mount 300 compression synthesized output signal, a pivot mount 300 tilt deflection synthesized output signal, and a pivot mount 300 tip deflection synthesized output signal represented by a transformation matrix equation, such as equation (10) described above. The synthesized output signals may be provided as inputs to dynamic control enable logic 1816. More particularly, dynamic control enable logic 1816 may observe the one or more synthesized output signals to determine that a contact disturbance 1812 has occurred in one or more of a tip, tilt, or z-direction. For example, if a non-zero compression signal is synthesized by signal conditioning and combination logic 1815 that exceeds predetermined limits, dynamic control enable logic 1816 may recognize the contact disturbance 1812.

In response to observing that a contact disturbance 1812 exists, dynamic control enable logic 1816 may close respective outer loops, each of which may be configured to provide output commands to modify the positional command of primary input 1802. Thus, closing the outer loops may drive the actuators to achieve a desired state of pressure and orientation, rather than driving them to achieve an initial position command. For example, if dynamic control enable logic 1816 observes that a compression contact disturbance 1812 exists, z-actuator outer loop 1818 may be closed to respond to the contact disturbance 1812 by adjusting a z-actuator. Likewise, dynamic control enable logic 1816 may respond to tip deflection signals or tilt deflection signals by enabling x-actuator outer loop 1820 or y-actuator outer loop 1822, respectively.

Deflection and compression feedback signals may be passed from signal conditioning and combination logic 1815 as synthesized output signals to respective outer loops for comparison with deflection command inputs 1840 provided to respective outer loops. In an embodiment, pivot mount 300 deflection command inputs 1840 may correspond to a desired pressure distribution across pivot mount 300 or MPA 103. Thus, pivot mount 300 deflection command inputs 1840 may represent tip deflection, tilt deflection, and z-compression targets of pivot mount 300. These targets may be compared to the synthesized output signals from signal conditioning and combination logic 1815, which indicate an instantaneous pressure distribution across pivot mount 300, to determine a difference. The difference, if any, may then be fed as an error signal to drive respective transfer head assembly 200 actuators. For example, if tipping of pivot mount 300 is sensed as a contact disturbance 1812 and dynamic control enable logic 1816 observes that the tipping exceeds an allowable amount, x-actuator outer loop 1820 may be closed and the tipping deflection signal may be compared with a pivot mount 300 tip deflection command 1840 to generate a motion control signal that will tip pivot mount 300 toward a desired stress state. The motion control signal may be fed to a servo filter and passed through inverse kinematics calculations to generate an outer loop command output 1830. In an embodiment, the motion control signal may also be added with other transfer head assembly motion control signals at one or more of motion summation nodes 1850. This may be the case, for example, when movement of multiple actuators is required to cause tipping.

In order to close the control loop, the outer loop command outputs 1830 may be combined with primary input 1802 and passed back into actuator inner loops. For example, a tipping outer loop command 1830 may be summed with primary input 1802 for an x-actuator and passed through x-actuator inner loop 1804, thereby controlling an x-actuator in such a manner that pivot mount 300 tips toward a physical state of more even pressure distribution. Respective outer loop commands may be passed through to any actuator inner loop for which a contact disturbance 1812 was sensed.

The above control methodology may be performed and repeated until the transfer head assembly 200 is moved to a location at which pressure distribution across pivot mount 300, and hence MPA 103, is uniform and achieves a desired amount of pressure. Thus, transfer head assembly 200 may be controlled to bring an array of electrostatic transfer head array 115 on MPA 103 into contact with an array of micro devices on a mating substrate. Using the control system described above, if alignment between MPA 103 and the mating substrate is not initially perfect, which would be true of almost every transfer operation, pressure distribution control may be implemented to fine tune the alignment. The control methodology may be performed quickly, e.g., on the order of about 50 ms to sense a contact disturbance 1812, enable the appropriate outer loop(s), and feed appropriate outer loop control commands to actuators, and thus, complete contact may be rapidly achieved between an electrostatic transfer head array 115 and an array of micro devices, enabling efficient transfer between a carrier substrate and a receiving substrate.

Referring now to FIG. 18B, a schematic illustration is provided for a method of generating a synthesized output signal in an embodiment. As illustrated, feedback signals 1814 are received by a signal conditioning and combination logic 1815, which combines the incoming feedback signals 1814 from the pivot mount 300 and generates synthesized output signals. In the simplest case, feedback signals received from the pivot mount (e.g. from sensors 1-24 described above with regard to FIG. 16) are linearly combined by multiplication with a transformation matrix to form a set of output measurements (synthesized output signals).

Referring to the embodiment illustrated in FIG. 18C, in a more complex implementation, correlated sets of strain sensors may be checked for signal quality. As illustrated, feedback signals 1814 are received by a signal conditioning and combination logic 1815. At 1815A, the feedback signals 1814 are checked to determine if they are within a predefined normal operating range. Sensors (including gages 320, 340) that are outside of the normal operating range are flagged as failed sensors. Failed sensor signals may then be rejected requiring a change in the transformation matrix. At 1815B signals are checked for variation within the normal operating range. Sensors (including gages 320, 340) with variation that is greater or less than a normally operating sensor are flagged as failed sensors. Based on the sensors flagged as failed, a transformation matrix is selected that is able to synthesize the outputs from the remaining signals, and the transformation matrix is used to convert the resulting sensor signal vector into synthesized output signals (position measurement output) at 1815C. In this way synthesized output signals are maintained at a reduced signal to noise ratio rather than sensor failure causing output failure.

Referring to FIG. 19, a flowchart illustrating a method of aligning an MPA 103 coupled with a pivot mount 300 on a transfer head assembly 200 relative to a target substrate is shown in accordance with an embodiment. The method may be performed, e.g., during a pick-up or a placement operation as micro devices are transferred from a carrier substrate to a receiving substrate. At operation 1902, mass transfer tool 100 moves transfer head assembly 200 along a z-axis toward a target substrate, e.g., carrier substrate held by carrier substrate holder 104 or receiving substrate held by receiving substrate holder 106, according to primary input 1302. More specifically, the MPA 103 and pivot mount 300 are moved toward the target substrate along the z-axis. Movement of MPA 103 along z-axis 510 may be achieved by actuating various actuators of mass transfer tool 100 or a substrate holder.

Initially, there may be no compressive loading applied to MPA 103 or pivot mount 300. This initial state may correspond to a range of travel over which array of micro devices are physically separated from the electrostatic transfer head array. During this travel, MPA 103 and the target substrate may have misaligned surfaces, but there may be no indication of this misalignment since the pressure distribution state of pivot mount 300 may be uniform, i.e., all strain sensing elements may be outputting signals indicating zero strain.

At operations 1904 and 1906, an electrostatic transfer head in the electrostatic transfer head array 103 may contact a micro device while other electrostatic transfer heads may remain separated from corresponding micro devices. That is, contact may be made while MPA 103 is misaligned with the target substrate. This positional misalignment may be sensed as uneven pressure distribution in pivot mount 300. For example, a first strain output value from one strain sensing element 320 on pivot mount 300 and a different second strain output value from another strain sensing element 320 in pivot mount 300 may differ. The strain signals may be provided as feedback signals 1814 and conditioned and combined by into synthesized output signals (e.g. tip deflection, tilt deflection, and compression signals) by signal conditioning and combination logic 1815 indicating a contact disturbance 1812.

Dynamic enable control logic 1816 may observe that the contact disturbance 1812 exists, and depending upon the level of contact disturbance 1812, may activate actuator outer loops to determine driving signals for actuating various actuators of transfer head assembly 200 in order to adjust an orientation of MPA 103 such that pressure distribution across pivot mount 300 is uniform. For example, at operation 1908, in response to the tip signal being recognized as a contact disturbance 1812 above a threshold, x-actuator outer loop 1820 may feed command signals 1830 to x-actuator inner loop 1804 in order to actuate an x-actuator to tip MPA 103 about remote rotational center. Similarly, at operation 1910, in response to the tilt deflection signal being recognized as a contact disturbance 1812 above a threshold, y-actuator outer loop 1822 may feed command signals to y-actuator inner loop 1806 in order to actuate a y-actuator 708 to tile MPA 103 about remote rotational center.

At operation 1912, in response to actuation of the x- and y-actuators based on the tip and tilt deflection signals MPA 103 may be rotated into alignment with the target substrate. Furthermore, with remote rotational center co-located with the contact surface of MPA 103, the electrostatic transfer head array 115 may experience pure rotation about remote rotational center. Thus, as MPA 103 is aligned with the target substrate, the electrostatic transfer head array 115 may experience minimal parasitic lateral motion and micro devices may remain undamaged.

Actuation of transfer head assembly 200 according to synthesized output signals (tip, tilt, and z-compression signals) may continue until the electrostatic transfer head array 115 is in contact with micro devices on the target substrate. More particularly, actuation may continue until primary output 1810 is within the limits set by primary input 1802, at which point actuation may be stopped. As discussed above, primary output 1810 may be a positional output that is modified to reach a desired pivot mount 300 state. For example, actuation of transfer head assembly 200 may continue until primary positional input is achieved and/or pressure distribution across pivot mount 300 is uniform.

After contact between the electrostatic transfer head array 115 and the micro devices is made, a voltage may be applied to the electrostatic transfer head array 115 to create a grip pressure on the array of micro devices. An electrostatic voltage may be applied to electrostatic transfer head array 115 compliant voltage contacts 316 and voltage contacts 120. Additional electrical contacts and connectors may be integrated within transfer head assembly 200 and powered by voltage supplies based on control signals from computer 108. For example, computer 108 may implement a control algorithm instructing that electrostatic transfer head array 115 be activated if a predefined deformation is simultaneously sensed by each strain sensing element on pivot mount 300 during a pick up process. As a result, the array of electrostatic transfer head array 115 may apply a gripping pressure to the array of micro devices after the entire array surface is in contact and uniform pressure is applied across the array.

After gripping the micro devices with electrostatic transfer head array 115, the micro devices may be picked up from carrier substrate. During pick up, the electrostatic voltage supplied to the electrostatic transfer head array 115 may persist, and thus, the array of micro devices may be retained on the electrostatic transfer head array 115 and removed from the carrier substrate.

During the pick up operation, a heating element may direct heat toward pivot mount 300 and/or MPA 103. Thus, the micro devices may be heated through contact with electrostatic transfer head array 115 on MPA 103 during pick up. For example, a heating element adjacent to pivot mount 300 may be resistively heated to transfer heat to MPA 103, and thus, to the micro devices through the electrostatic transfer head array 115. Heat transfer may occur before, during, and after picking up the array of micro devices from carrier substrate.

Although a pick up process is described in relation to FIG. 19, a similar methodology may be used to control the placement of micro devices onto a receiving substrate, such as a display substrate, held by receiving substrate holder 106. For example, as the micro devices are gripped by the electrostatic transfer head array 115, mass transfer tool 100 may move the MPA 103 over a receiving substrate, and align MPA with a target region of the receiving substrate. MPA 103 may be advanced toward, and aligned with, the receiving substrate using the control sequence described above until the array of micro devices held by the electrostatic transfer head array 115 are placed in uniform contact with the target region. Uniform contact may be inferred by sensing a strain state of pivot mount 300. Subsequently, voltage may be removed from the electrostatic transfer head array 115 to release the micro devices onto the receiving substrate and complete the transfer operation.

Referring to FIG. 20, a schematic illustration of a computer system is shown that may be used in accordance with an embodiment. Portions of embodiments are comprised of or controlled by non-transitory machine-readable and machine-executable instructions that reside, for example, in machine-usable media of a computer 108. Computer 108 is exemplary, and embodiments may operate on or within, or be controlled by a number of different computer systems including general purpose networked computer systems, embedded computer systems, routers, switches, server devices, client devices, various intermediate devices/nodes, stand-alone computer systems, and the like. Furthermore, although some components of a control system, e.g., signal conditioning and combination logic 1815 and dynamic control enable logic 1816, have been broken out for discussion separately above, computer 108 may integrate those components directly or include additional components that fulfill similar functions.

Computer 108 of FIG. 20 includes an address/data bus 2002 for communicating information, and a central processor 2004 coupled to bus 2002 for processing information and instructions. Computer 108 also includes data storage features such as a computer usable volatile memory, e.g. random access memory (RAM) 2006, coupled to bus 2002 for storing information and instructions for central processor 2004, computer usable non-volatile memory 2008, e.g. read only memory (ROM), coupled to bus 2002 for storing static information and instructions for the central processor 2004, and a data storage device 2010 (e.g., a magnetic or optical disk and disk drive) coupled to bus 2002 for storing information and instructions. Computer 108 of the present embodiment also includes an optional alphanumeric input device 2012 including alphanumeric and function keys coupled to bus 2002 for communicating information and command selections to central processor 2004. Computer 108 also optionally includes an optional cursor control 2014 device coupled to bus 2002 for communicating user input information and command selections to central processor 2004. Computer 108 of the present embodiment also includes an optional display device 2016 coupled to bus 2002 for displaying information.

The data storage device 2010 may include a non-transitory machine-readable storage medium 2018 on which is stored one or more sets of instructions (e.g. software 2020) embodying any one or more of the methodologies or operations described herein. For example, software 2020 may include instructions, which when executed by processor 2004, cause computer 108 to control mass transfer tool 100 or remote center robot 500 according to the control scheme described above for aligning an MPA 103 with a target substrate. Software 2020 may also reside, completely or at least partially, within the volatile memory, non-volatile memory 2008, and/or within processor 2004 during execution thereof by computer 108, volatile memory 2006, non-volatile memory 2008, and processor 2004 also constituting non-transitory machine-readable storage media.

In utilizing the various aspects of the embodiments, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for forming a pivot mount with integrated strain sensing elements. Although the embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the appended claims are not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as embodiments of the claims useful for illustration. 

What is claimed is:
 1. A pivot mount comprising: a pivot platform; a base; a primary spring arm fixed to the pivot platform at a primary inner root, fixed to the base at a primary outer root, and characterized by a corresponding primary axis length spanning between the primary outer root and primary inner root; and a secondary spring arm fixed to the pivot platform at a secondary inner root, fixed to the base at a secondary outer root, and characterized by a secondary axis length spanning between the secondary outer root and secondary inner root; and a strain sensing element along the secondary spring arm; wherein the secondary spring arm is characterized as having a lower stiffness than the primary spring arm.
 2. The pivot mount of claim 1, wherein the primary axis length is greater than the secondary axis length.
 3. The pivot mount of claim 2, wherein an average width of the primary spring arm along the primary axis length is wider than an average width of the secondary spring arm along the secondary axis length.
 4. The pivot mount of claim 3, wherein an average thickness of the primary spring arm along the primary axis length is substantially equal to an average thickness of the secondary spring arm along the secondary axis length.
 5. The pivot mount of claim 3, wherein the primary spring arm and the secondary spring arm are formed of a same material.
 6. The pivot mount of claim 3, wherein the secondary spring arm includes a switch-back along the secondary axis length such that a first beam segment and a second beam segment of the secondary spring arm immediately adjacent the switch-back are parallel to each other.
 7. The pivot mount of claim 6, wherein the strain sensing element is a first strain sensing element at the first beam segment of the secondary spring arm; and further comprising a second strain sensing element at the second beam segment of the secondary spring arm.
 8. The pivot mount of claim 7, further comprising a first reference gauge adjacent the first strain sensing element at the first beam segment of the secondary spring arm, and a second reference gauge adjacent the first strain sensing element at the first beam segment of the secondary spring arm.
 9. The pivot mount of claim 6, further comprising a plurality of switch-backs along the secondary axis length.
 10. The pivot mount of claim 9, wherein the plurality of switch-backs along the secondary axis length are parallel.
 11. The pivot mount of claim 9, wherein the second beam segment is longer than the first beam segment.
 12. The pivot mount of claim 3, wherein the secondary spring arm includes a plurality of switch-backs along the secondary axial length, a plurality of beam segments of a first length along the secondary axial length, and a beam segment of a second length longer than the first length along the secondary axial length.
 13. The pivot mount of claim 1, wherein an average thickness of the primary spring arm along the primary axis length is greater than an average thickness of the secondary spring arm along the secondary axis length.
 14. The pivot mount of claim 2, comprising: a pair of secondary spring arms laterally between a pair of primary spring arms, wherein each of the secondary spring arms is characterized as having a lower stiffness than each of the primary spring arms.
 15. A transfer tool comprising: an articulating transfer head assembly; a pivot mount, mountable onto the articulating transfer head assembly comprising: a pivot platform; a base; a primary spring arm fixed to the pivot platform at a primary inner root, fixed to the base at a primary outer root, and characterized by a corresponding primary axis length spanning between the primary outer root and primary inner root; and a secondary spring arm fixed to the pivot platform at a secondary inner root, fixed to the base at a secondary outer root, and characterized by a secondary axis length spanning between the secondary outer root and secondary inner root; and a strain sensing element along the secondary spring arm; wherein the secondary spring arm is characterized as having a lower stiffness than the primary spring arm; and a micro pick up array mountable onto the pivot platform of the pivot mount, the micro pick up array including an array of transfer heads.
 16. The pivot mount of claim 15, wherein the secondary spring arm includes a plurality of switch-backs along the secondary axial length, a plurality of beam segments of a first length along the secondary axial length, and a beam segment of a second length longer than the first length along the secondary axial length.
 17. The transfer tool of claim 15, wherein each transfer head has a localized contact point characterized by a maximum dimension of 1-100 μm in both x- and y-dimensions.
 18. The transfer tool of claim 15, wherein the primary axis length is greater than the secondary axis length.
 19. The transfer tool of claim 18, wherein an average width of the primary spring arm along the primary axis length is wider than an average width of the secondary spring arm along the secondary axis length.
 20. The transfer tool of claim 15, wherein the pivot platform comprises a plurality of compliant voltage contacts, and the micro pick up array comprises a plurality of voltage contacts arranged to mate with the plurality of compliant voltage contacts of the pivot platform.
 21. The transfer tool of claim 15, wherein an average thickness of the primary spring arm along the primary axis length is greater than an average thickness of the secondary spring arm along the secondary axis length. 